A. Introduction
The present invention relates to the field of oil well logging, and more particularly, to the field of well logging which is intended to determine the percent of residual oil remaining in borehole adjacent formations which have been subjected to primary and secondary production procedures.
Once primary oil production from a well is complete, it is known to stimulate further well production by forcing water into a pay zone to sweep or flush oil out of that zone. Such production, usually termed "secondary production", may still leave substantial amounts of residual oil in the pay zone. Even after a water flood, it is not unusual for 20-40% of the oil originally contained in the pay zone to remain. In other pay zones, however, much lower residual oil percentages remain after secondary production.
It is important to know the amount of residual oil remaining in a pay zone after a secondary production. Depending on the amount of oil remaining in the pay zone, it may be economical to plan a tertiary flood wherein other fluids are pumped into the well to flush additional amounts of oil from the pay zone. Such tertiary fluids comprise detergents, carbon dioxide, and or oil solvents.
Many techniques have been suggested for making residual oil determinations. These methods include conventional coring, pressure coring, back flow tracing and other logging techniques. For a review of many of these techniques, please refer to "How Should We Measure Residual-Oil Saturation?", by R. E. Wyman, Bulletin of Canadian Petroleum Geology, Vol. 25, No. 2 (May 1977), pages 233-270, which is hereby incorporated by reference. As reviewed by Mr. Wyman, basic logging methods include resistivity, pulsed neutron capture, nuclear magnetism, carbon/oxygen, gamma radiation, and dielectric logging techniques. In another review article entitled "Review of Petroleum Oil Saturation & Its Determination", by E. C. Donaldson and M. E. Crocker, U.S. Dept. of Energy, Technical Information Center, publication #BERC/RI77/15 (distribution category UC 92a), many additional methods of determining residual oil saturation are suggested. In the area of well logging, resistivity, spontaneous potential, focusing electrode, induction, sonic, formation density, neutron, gamma-ray, thermal neutron decay-time, nuclear magnetic, and log-inject-log logging techniques are described. After reviewing the above mentioned techniques and several others, Messrs. Donaldson and Crocker conclude:
"No method for the determination of the residual oil saturation is so general that it can be used in all situations that arise in field operations. Best results are obtained from selected multiple tests in a single hole. Wyman [supra.]. . . states that the nuclear magnetic log is the most accurate of all of the logging methods; however, its depth of investigation is limited and it can only be used in an open hole. In many respects, the log-inject-log procedure using the neutron capture tool has the most general range of application since it can be used in either cased or uncased wells." Donaldson et al, supra, at page 33.